1. Field of the Invention
This invention relates to a magnetic coupling assembly for a propeller-operated fluid flowmeter.
2. State of the Art
It is known to employ magnetic coupling assemblies in the drive means connecting the propeller of a fluid meter to the register of the meter. For example, according to U.S. Pat. No. 3,398,577 to Kovats et al, a magnet is coupled to the shaft of the propeller to rotate with the propeller, and a second magnet is coupled to the shaft of the register mechanism to operate the register. A magnetic flux keeper is interposed between the first and second magnets to transfer the magnetic fields therebetween. Thus, the first and second magnets are located within the magnetic fields of one another so that motion of one magnet induces motion of the other.
Further, according to this patented system, the magnet coupled to the propeller shaft includes a single north and a single south pole; and likewise the magnet coupled to the register includes a single north and a single south pole. In operation, as the driving magnet on the propeller shaft rotates, the magnetic flux induces the magnet coupled to the register shaft to rotate due to attraction of opposite poles and repulsion of like poles. The driving magnet rotates one revolution for each revolution of the driven magnet.
For proper operation of a flowmeter employing magnetic coupling it is important that the register magnet rotate without "slipping" relative to the driving magnet. That is, if the propeller were able to rotate without causing commensurate rotation in the register unit, then inaccurate measurements of flow would be recorded. It has been found that liquid streams often contain substantial discontinuities such as air pockets or large particles of material. Such discontinuities can cause the propeller to experience substantial accelerations when they pass thereby. According to prior art systems of the type described in the aboveidentified patent, substantial accelerations of the propeller have been found to result in "slipping" between the driving magnet and the driven magnet.
It is also known that the flow rate of the liquid can be reduced due to the torque required to turn the propeller. In very small pipes this torque can be a substantial percentage of the energy in the flowing fluid. Therefore it is desirable to minimize the torque required to turn the propeller and drive the register.